It would be nice with an ID so you know who sent what message without having to believe the client that sent the message. For example in a chat it's easy to say that some other client sent a chat message but just altering the message sent.

"Moru: Blablabla" - realy sent from "haxor" guy

Also some way of noticing when a client disconnected or to kick a client.

Some way of setting what port the server is listening on and ofcourse the client needs to know what port to connect to if it's not the standard port.